This invention relates to an ion implantation process, particularly ion implantation of phosphorus into a semiconductor substrate using ion implanting equipment having mass separation.
Ion implantation is an important technique used in the manufacture of semiconductor devices. Ion shower systems are commonly used for implantation into a larger area of semiconductor material. Alternatively, an ion implantation apparatus is known using mass-analysed ion implantation, which effects selection by mass of ions desired for implantation into the semiconductor substrate. A mass spectrometer typically performs the mass separation using a static magnetic field generated by an electron magnet, wherein selected ion species are obtained by controlling the electric current in the magnetic coil so that the selected ion species pass through a resolving slit. Mass separation is suitable for narrow ion implantation beams, which can be scanned to cover a large area semiconductor layer.
One common donor atom implanted into semiconductor material to create an n-type doped semiconductor is phosphorus, and it is known to use an ion source of phosphine (PH.sub.3) for phosphorus doping. A problem with conventional ion shower implantation is that a number of ion species are implanted into the semiconductor substrate during the implantation process. Mass separation systems may also not have the resolution to discriminate between individual ion species. For example, phosphorus hydride ions (present in ionised phosphine) have very similar atomic masses to phosphorus ions, so that accurate mass analysis is required in order to separate the phosphorus ion species. Implantation of some hydrides may not be considered to be a problem, particularly in the case of poly-silicon or single crystal silicon semiconductor substrates. However, for doping of amorphous silicon layers the effects of hydride impurities is more pronounced. It has been recognised that implantation of hydrogen ions should be avoided, and the ion implantation system described in U.S. Pat. No. 4,533,831 seeks to avoid implantation of hydrogen ions. This is achieved generally by separating heavier ions from lighter ions. U.S. Pat. No. 4,533,831 does not eliminate the implantation of phosphorus hydride ions.